petee's handbook of zooloot. 35 



The King- Crabs constitute a fourtli order, leading naturally from 

 the Malacostracous to the Entomostracous Crustacea, and for this 

 Dr. Grerstaecker adopts the name of Poecilopoda, applied by Latreille 

 to a heterogeneous assemblage, in which these animals figure together 

 with ArguluSj Caligus, Anthosoma, and several other parasitic genera. 

 Wliy he has rejected Latreille's term Xyphosura for the King-Crabs, 

 or rather sunk it into a family name does not appear ; it is certainly 

 the most characteristic name for the order, and the change is by no 

 means an advantageous one. 



The fifth order, Branchiopoda, receives from our author a wider 

 extension than is given to it by Milne-Edwards, embracing the Fossil 

 Trilobites and the Cypridiform Crustacea, in addition to the Phyl- 

 lopoda and Cladocera of Latreille. With regard to the precise 

 sytematic station of the Trilobites (which Dr. Gerstaecker erro- 

 neously describes (p. 395) as " the oldest representatives not only of 

 the Anthropoda, but of all animal organisms") we certainly possess no 

 positive evidence, and although Burmeister's investigations have shown 

 that their nearest allies in existing nature are the Phyllopoda, they 

 nevertheless present characters which, taken in conjunction with 

 their limited distribution in time, would seem to justify our regard- 

 ing them as a distinct order. The difficulty of placing the Trilobites 

 in a definite position is, however, only a negative one, arising from 

 our ignorance of those parts from which the essential characters of 

 the orders are derived, but the Ostracoda have evidently presented 

 our author with a positive difficulty, which has interfered materially 

 with his definition of the order Branchiopoda — a difficulty which he 

 has but imperfectly got over, by assuming that the two pairs of 

 branchiferous footjaws in these Crustacea are in reality to be re- 

 garded as belonging to the series of abdominal feet. This is a point 

 which may be cleared up by future researches, — in the meanwhile it 

 is certainly better to place the Ostracoda in the same order with 

 the very analogous Daphnidaa, than to adopt the only other course, 

 that of establishing a distinct order for this small group. This 

 indeed is the only alternative open to us, for Dr. Gerstaecker's sixth 

 order, to which he restricts the term Entomostraca, must be re- 

 garded as a perfectly natural group. In it he includes, besides the 

 Copepoda, the whole of the parasitic Entomostraca of authors, form- 

 ing a group which it is perhaps difficult to characterise satisfactorily, 

 but which, from the close similarity in the young animals, and the 

 agreement in many important points of the life-history of its mem- 



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